Lake Superior Is Certainly Dry Suit Diving
As a Master Diver who's spent about 3 months of my life under water, I'll take a stab at helping you with your novel.
Lake Superior is very cold water, certainly requiring a SCUBA diver to use a dry suit. (You can Google that term to get a better definition.)
Superior is a very deep lake, over 1,000 feet deep in the middle. Certainly no one would be rescue diving on any ship sunk in water that deep. There are lots of wrecks around Isle Royale and the shorelines and a few shallow shoals of Superior, about the only places where rescue diving would be reasonable.
As mentioned above, there are ways that a SCUBA diver could extend the length and depth of his dives--the dry suit for warmth, air mixes for extending the length of the dive, etc. But generally, a diver on SCUBA is going to be limited to relatively short dives--five to ten minutes max at depths less than 150 feet. For authenticity of your novel, you should probably research the term "Navy Dive Tables". They'll give you the4 combinations of depth and time that can be dived safely on SCUBA.
Hope this helps.
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